When a magnetic material, e.g., the magnetic film of a recording disk, is placed in a magnetic field H, a magnetic flux M is induced in the material. (For purposes of background explanation, reference is made to the M-H hysteresis loop which is shown in FIG. 9.) The loop closure points in an M-H hysteresis loop (such as the one illustrated in FIG. 9 herein) define the positive and negative magnetic field values H.sub.s at which reflux saturation occurs. If the field is varied from H.sub.s to zero, the material retains a characteristic flux density M.sub.r, or remanence, which measures the ability of the material or hold magnetic flux in the absence of an external magnetic field. Operationally, remanence determines the signal amplitude which can be read from an isolated pulse stored in the medium--the greater the remanence, the greater the signal amplitude which can be detected in a reading operation.
A second important property of a recording medium is its intrinsic coercivity H.sub.c, defined as the magnetic field required to reduce the remanence magnetic flux to O, i.e., the field required to erase a stored information bit in the medium. With reference to FIG. 9, H is defined as the measured magnetic field at M=0. It can be appreciated that higher coercivity in a medium allows adjacent recorded bits to be place more closely together without mutual cancellation. Accordingly, higher coercivity in a magnetic medium is associated with higher information storage density.
Other important magnetic properties are loop squareness, and the ratio of coercivity to saturation field, i.e., H.sub.c /H.sub.s. As can be appreciated with reference to FIG. 9, as H.sub.s becomes smaller (approaches H.sub.c), it takes less field strength to switch or "write" the medium. In practical terms, this means that when a new signal is written over an old signal, the ratio of old signal residual to new signal is relatively small. This ratio is also referred to as overwrite, a small overwrite ratio including good writability. In summary, high remanence and coercivity and high hysteresis-loop squareness contribute importantly to signal strength, storage density, and overwrite characteristics in a magnetic recording medium.
Considerable effort has been devoted in the prior art to the preduction of magnetic recording media having the desired properties discussed above. One method which has received increasing attention involves vapor deosition of an ion-bombarded target metal, or sputtering onto a substrate. In the usual sputtering system, a pair of disk-like substrates, carried in a side-by-side arrangement on a pallet, is moved through a succession of sputtering stations, in a front-to-back direction, to produce one or more underlayers, an outer magnetic thin film, and a protective coating. The overall method provides efficient, high throughput production of multi-layered thin-film media.
Despite these advantages, sputtering systems of the type mentioned above have not been entirely satisfactory, in that the sputtered layer may show significant crystal anisotropy and/or variations in layer thickness. Both types of surfaces nonuniformities lead to angular variations in magnetic signal properties, particularly at outer-track regions of a magnetic disk. As will be seen below, signal-amplitude variations of up to about 25%, as measured at an inner-diameter recording track, and up to about 40%, as measured at an outer-diameter recording track, are typical in magnetic recording disks formed in sputtering systems of the type described above.
In theory, it should be possible to eliminate crystal anisotropy and variations in film thickness in a sputtering operation by rotating the substrates as they pass through each of the sputtering stations. However, it would be relatively difficult and expensive to adapt existing types of sputtering systems to provide simultaneous linear and rotational substrate movement through the various sputtering stations. An alternative approach which is compatible with the design of existing commercial sputtering machines would be to partition each sputtering target into a number of smaller target regions by placing multiple shields or baffles between the target and the region where deposition occurs. These baffles would act to prevent all but direct, high-angle deposition from the target onto the substrate. A number of baffle configurations, including a multi-web lattice or a plurality or relatively close-packed cylinders, would be suitable. Although this approach would result in a sputtered layer having an isotropic crystal structure and relatively uniform thickness, the time and amount of target material needed to form the layer would be relatively great, since a major portion of the sputtered material would be deposited on the walls of the baffles. Maintenance problems relative to removing deposited material from the baffles regularly would be considerable, as well.